1. Field
One embodiment of the present invention relates to a projection optical system, exposure apparatus, and exposure method, particularly to a projection optical system suitable for an exposure apparatus used in manufacturing micro-devices such as a semiconductor device and a liquid crystal display device, for example, in a photolithography process.
2. Description of the Related Art
In a photolithography process for manufacturing a semiconductor device and the like, an exposure apparatus has been used, which projects a pattern image of a mask (or reticle) onto a photosensitive substrate (a wafer, glass plate and the like, on which photoresist is applied) to effect exposure thereof through a projection optical system. As the integration density of semiconductor devices increases, the resolving power (or resolution) required in the projection optical system is more and more increased in the exposure apparatus.
Therefore, in order to meet the requirement on the resolving power of a projection optical system, it is necessary to make wavelength λ of illuminating light (or exposure light) shorter and to increase the numerical aperture NA on the image side of the projection optical system. Specifically, the resolution of a projection optical system is represented as k·λ/NA (where k is a process coefficient). In addition, the image-side numerical aperture NA is represented as n·sin θ where n is the refractive index of a medium (typically gas such as air) between the projection optical system and a photosensitive substrate, and θ is the maximum angle of incidence onto the photosensitive substrate.
In this case, an attempt to increase the image-side numerical aperture by increasing the maximum angle of incidence θ results in an increased angle of incidence onto the photosensitive substrate and an increased angle of emergence from the projection optical system; this increases reflection losses on an optical plane, and therefore, a large effective image-side numerical aperture cannot be achieved. Therefore, an immersion technique has been proposed in International Publication WO2004/019128, which increases in the image-side numerical aperture by filling an optical path between a projection optical system and a photosensitive substrate with a medium such as liquid having a high refractive index.